Known laser processing apparatuses for processing sheet metal employ, as a laser beam source, a carbon dioxide (CO2) gas laser oscillator, a YAG laser oscillator, or a fiber laser oscillator. The fiber laser oscillator is superior in beam quality to the YAG laser oscillator and has advantages of providing a very high oscillation efficiency and the like. Due to this, the fiber laser processing apparatus using the fiber laser oscillator is used in industrial fields, in particular, in sheet metal processing (cutting, welding, and the like).
Developed in recent years is a direct diode laser (DDL) processing apparatus that uses, as a laser beam source, a direct diode laser oscillator. The DDL processing apparatus employs a plurality of laser diodes (LDs), superimposes laser beams into a multiple-wavelength laser beam, and transmits the laser beam through a transmission fiber to a processing head. The laser beam emitted from an end face of the transmission fiber is concentrated through and irradiated from collimating and condensing lenses onto a workpiece.
In connection with the fiber laser processing apparatus, it is known, as exemplified in Published PCT International Application No. WO2010/034603 (Patent Literature 1), to cut a thick plate by condensing a unique-wavelength laser beam through a multifocal lens to form a plurality of focal points in a thickness direction of the workpiece.